Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures
Original notes — ready CBSE (NCERT, English Medium) • Class 8 • Science • Book: Curiosity
Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures — EG's own notes, in simple words
Book: Curiosity (CBSE, Class 8) | Chapter 8 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): one kind of particle alone, two kinds joined together, and different kinds simply mixed. A general picture about elements, compounds and mixtures, not a diagram from the book.
Sorting different objects into separate groups. A general picture about separating mixtures, not a book scene.
What the chapter is about
All matter is not the same. Some substances are made of only one kind of particle, some are made of two or more kinds joined together, and some are just different substances mixed together. Learning to tell these apart is the key to this chapter.
An element is a pure substance made of only one kind of atom — for example, iron, oxygen and gold. A compound is a pure substance formed when two or more elements join together chemically in a fixed ratio — for example, water, which is made of hydrogen and oxygen. A compound has properties different from the elements that made it.
A mixture is different: two or more substances are simply mixed, not joined chemically, and they can be present in any amount. Air, salt water and a salad are mixtures. The parts of a mixture keep their own properties and can usually be separated by simple physical methods.
What you should learn
Notice the difference between an element, a compound and a mixture — and why compounds have fixed ratios while mixtures do not.
Key Terms
- Element — A pure substance made of only one kind of atom.
- Compound — A pure substance formed when two or more elements combine chemically in a fixed ratio.
- Mixture — Two or more substances simply mixed together, not chemically combined, in any proportion.
- Atom — The smallest unit of an element that keeps the properties of that element.
- Molecule — Two or more atoms joined together; a molecule of a compound contains atoms of different elements.
Remember
- Elements contain only one kind of atom.
- Compounds form by chemical combination in a fixed ratio and have new properties.
- Mixtures are physical combinations in any proportion; parts keep their properties.
- Mixtures can usually be separated by simple physical methods.
Exam Tip / Common Mistake
Common mistake: calling air a compound. Air is a mixture — its gases are not chemically joined and their amounts can vary. Water, by contrast, is a compound with a fixed ratio of hydrogen to oxygen.
Chart / Table: Elements, compounds and mixtures
| Point | Detail |
|---|---|
| Element | One kind of atom; e.g. iron, oxygen, gold |
| Compound | Elements chemically joined in fixed ratio; e.g. water |
| Mixture | Substances physically mixed in any ratio; e.g. air, salt water |
| Separation | Mixtures: physical methods; compounds: chemical methods |
| Properties | Compound: new properties; mixture: parts keep their properties |
Practice set — try it yourself
- What is an element? Give two examples.
Hint: Think of one kind of atom. - How is a compound different from a mixture?
Hint: Think about chemical joining and fixed ratio. - Is air an element, a compound or a mixture? Why?
Hint: Think about its gases and their amounts. - Why does water have properties different from hydrogen and oxygen?
Hint: Think about chemical combination. - Name a method to separate a mixture.
Hint: Think of filtration or evaporation.
Questions with Answers
- Define element, compound and mixture with one example each.
An element is a pure substance of one kind of atom, like iron. A compound is elements chemically combined in a fixed ratio, like water. A mixture is substances physically mixed in any proportion, like air. - Why is water a compound but salt water a mixture?
Water is formed when hydrogen and oxygen combine chemically in a fixed ratio and gain new properties. Salt water is just salt dissolved in water — the salt and water are not chemically joined and can be separated by evaporation. - Can the parts of a mixture be separated easily? Explain.
Yes, usually by physical methods, because the parts are not chemically joined and keep their own properties. For example, sand can be separated from water by filtration. - Why can a compound be separated only by chemical methods?
Because its elements are joined by chemical bonds in a fixed ratio. Breaking those bonds needs a chemical change, not a simple physical method like filtering. - Is milk a pure substance or a mixture? Why?
Milk is a mixture. It contains water, fats, proteins and other substances mixed together, not chemically combined in a fixed ratio.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.